With reference to FIG. 1, a cordless power tool is illustrated and designated with reference numeral 1. The cordless power tool 1 ordinarily includes a clam shell type housing 2. The housing 2 includes a mechanism 3 to couple the housing 2 with a battery pack 4. The cordless power tool 1 includes electrical elements 5, typically included in a terminal block (not shown in FIG. 1), which couple with corresponding electrical elements 6 of the battery pack 4, also typically included in a terminal block (not shown in FIG. 1). The cordless power tool 1 includes a trigger 7, such as a trigger switch and which may be referred to herein as trigger 7, which is activated for energizing a motor 8 provided within the housing 2, as is well known in the art. Motor 8 may illustratively be a permanent magnet DC motor of the type conventionally used in cordless power tools. Normally, a plurality of battery cells 9 are disposed within the battery pack
A controller 10 may be provided in housing 2 for controlling motor 8. The cordless power tool may include a gear case 12 that couples motor 8 to an output, such as an adjustable chuck 14.
In certain cordless power tools, gear case 12 includes a transmission having multiple speed ranges. For example, certain cordless drills, hammer drills and hammers have three speed transmissions, such as the DEWALT XRP series of drills, hammer drills and hammer drills. One example is the DEWALT Heavy Duty XRP 18 V Hammer Drill, model DC989VA which is a variable speed hammer drill having a three speed transmission with high, medium and low speed ranges. The transmission has an output speed that ranges from 0-2000 rpm in the high speed range, an output speed that ranges from 0 to 800 rpm in the medium speed range, and an output speed that ranges from 0 to 500 rpm in the low speed range. These speed ranges may also be referred to in terms of their torque characteristics. For example, in a transmission having high, medium and low speed ranges these ranges may be referred to as the light, medium and heavy torque ranges, the light torque range corresponding to the high speed range, the medium torque range corresponding to the medium speed range and the low speed range corresponding to the high torque range.
Variable speed control in power tools is typically accomplished by pulse width modulating the voltage applied to the motor of the power tool and varying the duty cycle of the pulse width modulation to vary the speed of the motor. FIG. 2 shows a prior art motor control circuit 110 for controlling power to a motor 112 in a cordless power tool electrical system 114 (shown representatively by dashed box 114). Cordless power tool electrical system 114 is illustratively a variable speed system, such as would be used in a variable speed drill, hammer drill or hammer of the type discussed above. Motor 112 illustratively has a permanent magnet field and a wound armature. Motor control circuit 110 includes a power switch 116, illustratively a trigger switch, having main power contacts 118 and bypass contacts 122. It may also optionally have braking contacts 120 in which case main power contacts 118 and braking contacts 120 are linked so that they operate in conjunction with each other. Main power contacts 118 are normally open and braking contacts 120 are normally closed and both are break-before-make contacts. The normally open side of main power contacts 118 is connected to the negative terminal of a battery 124 and the common side of main power contacts 118 is connected to controller 126 of motor control circuit 110. Motor control circuit 110 also includes run power switching device 128 and free wheeling diode 130.
Run power switching device 128 is illustratively a N-channel MOSFET with its gate connected to an output of controller 126, its source connected to the common side of main power contacts 118 and its drain connected the common side of braking contacts 120 of trigger switch 116, to one side of the windings of motor 112 and to the anode of diode 130. As is known, MOSFETs have diodes bridging their sources and drains, identified as diode 132 in FIG. 1. The other side of braking contacts 120 is connected to the positive side of battery 124 as is the other side of the windings of motor 112 and the cathode of diode 130. Since motor 112 is illustratively a wound armature/permanent magnet field motor, the motor windings to which the drain of run power switching device 128 and the positive side of battery 124 are connected are the armature windings.
Controller 126 is illustratively a pulse width modulator that provides a pulse width modulated signal to the gate of run power switching device 128 having a set frequency and a variable duty cycle controlled by a variable resistance. The variable resistance is illustratively a potentiometer 119 mechanically coupled to trigger switch 116. In this regard, controller 126 can be a LM 555 and potentiometer, the LM 555 configured as a pulse width modulator having a set frequency and a variable duty cycle controlled by the potentiometer that is mechanically coupled to trigger switch 116.
In operation, trigger switch 116 is partially depressed, opening braking contacts 120 and closing, a split second later, main power contacts 118. This couples power from battery 124 to controller 126, to the source of run power switching device 128 and to bypass contacts 122 (that remain open at this point). Controller 126 generates a pulse width modulated signal at the gate of run power switching device 128, cycling it on and off. Run power switching device 128 switches power on and off to the windings of motor 112 as it cycles on and off. The duty cycle of the pulse width modulated signal, that is, how long it is high compared to how long it is low, provided at the gate of run power switching device 128 is determined by how far trigger switch 116 is depressed. (How far trigger switch 116 is depressed determines the variable resistance of the potentiometer 119 mechanically coupled to it that provides the variable resistance used to set the duty cycle of controller 126.) The duty cycle of the pulse width modulated signal determines the speed of motor 112. As trigger switch 116 is depressed further, bypass contacts 122 close, typically when trigger switch 116 is depressed to about the eighty percent level. When bypass contacts 122 close, power is connected directly from the battery 124 to the motor windings and the variable speed control provided by controller 126 and run power switching device 128 is bypassed. Motor 112 then runs at full speed.
Diode 130, known as a free wheeling diode, provides a path for the current in the windings of motor 112 when run power switching device 128 switches from on to off. Current then flows out of the motor windings at the bottom of motor 112 (as oriented in FIG. 1) through diode 130 and back into the motor windings at the top of motor 112 (as oriented in FIG. 1).